1. Field of the Invention
The present invention relates to a navigation system, installed in a vehicle, which displays the position and direction of the vehicle, map information of an area around the vehicle, etc., and, more particularly, to a navigation system which calculates coefficients of devices, such as a velocity sensor, for obtaining a travel distance based on the rotation of the tires of the vehicle.
2. Description of the Related Art
A known vehicle navigation system obtains travel distances and rotational angles by using a velocity sensor or a distance sensor and an angle sensor and, based on the travel distances and rotational angles, determines the position and direction of the vehicle. If necessary, the vehicle's positional data is corrected by comparing a travel path of the vehicle with map data. Further, if a GPS (Global Positioning System), which measures the absolute position of a vehicle, is available, the position measured by the navigation system can be corrected so as to equal the position measured by the GPS (GPS position).
This known navigation system obtains a travel distance by a velocity sensor which detects pulses or the like outputted in accordance with the rotation of the tires of the vehicle and by the number of the detected pulses multiplied by a coefficient which is used for converting a number of pulses to a distance, that is, a coefficient of the velocity sensor.
The GPS cannot always fully function; for example, a sufficient number of satellites are usually not available for positioning in the vicinity of tall buildings or high mountains. Therefore, in the above-described known navigation apparatus, in order to perform sufficiently precise positioning even in such areas where GPS is not usable, the positioning precision of dead reckoning performed by using a velocity sensor and an angular velocity sensor must be substantially high. In order to maintain high positioning precision of the dead reckoning system, a user must correct the coefficient of the velocity sensor if the size of tires changes because of, e.g., age deterioration or pressure changes in the tires, or because the tires are changed. However, a change in this coefficient greatly affects the positioning precision, and it is not easy for the user to perform such a delicate correction.
To avoid this problem, a method has been developed in which the velocity sensor coefficient is automatically corrected by using map data, more specifically, by comparing the length of a travel path of a vehicle which is obtained on the basis of various positions of the vehicles calculated by the navigation system with the corresponding distance separately obtained from the road data of the map data. FIG. 1 illustrates this method. While the length of a travel path 503 between two corners 501 and 502 is being compared with the distance 504 therebetween obtained from road data, the velocity sensor coefficient is automatically corrected so that the travel path length 503 becomes equal to the distance 504 based on road data.
However, in this method, positioning precision sometimes deteriorates. As shown in FIG. 2, if there are two streets 602, 603 which are close to each other in the vicinity of a corner 601 and have similar appearances and directions, one street may be mistaken for the other. If the street 603 is mistaken for the street 602 on a map when the velocity sensor coefficient is going to be automatically corrected, the navigation system calculates such a velocity sensor coefficient that a travel path length 604 becomes equal to the distance 605 to the street 603 instead of the distance to the street 602, thus increasing any error in the velocity sensor coefficient. Therefore, the positioning precision is deteriorated.
Further, Japanese Patent Unexamined Publication No. 2-212714 discloses a method in which a ratio is obtained between an integrated travel distance of a travelling object which is obtained by the GPS navigation method using satellites and another integrated travel distance which is obtained by an independent navigation method using a distance sensor or the like, and the output of a distance sensor is corrected by using this ratio as a correction parameter.
In this method, however, a travel distance obtained on the basis of outputs of the sensor is likely to deviate from the actual travel distance if the distance between positioning points is great and the travel path has curves. Thus, the method will likely fail to precisely correct sensor outputs.